This invention relates to the insulation of metallic conductors of electricity, and more particularly to the protection of insulating coatings from damage caused by corona discharges.
Insulation on high voltage electrical conductors is frequently exposed to corona discharges. Such discharges are the result of breakdown of gases in voids in the insulation or in air adjacent to the insulation, and can cause degradation of the insulating coating.
Numerous methods have been disclosed for protecting polymeric insulating coatings against the effects of corona discharge. They generally involve suspension of inorganic fillers in the polymer. Typical fillers include organometallic compounds of such elements as silicon, tin, lead, antimony, iron and nickel and inorganic oxides such as those of zinc, ferric iron and aluminum. Reference is made, for example, to U.S. Pat. Nos. 4,537,804, 4,760,296 and 4,935,302 and European patent application 287,814.
Recently, there has been interest in employing temperature-resistant polymers such as polyimides, polyetherketones, polyethersulfones and polyphthalamides as insulators for high voltage equipment. The incorporation of fillers such as mica in such polymers to protect against the effects of corona discharges has been attempted but is not entirely successful. Among the reasons are the unavailability of low cost solvents in which such polymers may be dissolved and the filler materials dispersed, and the fact that the high filler loadings, typically above 15% by weight, which are necessary to provide the desired protection against corona discharges are difficult or impossible to attain.
Studies have also been made of the incorporation of polyorganosiloxanes, hereinafter sometimes designated "silicones", in polymers to protect against corona discharge. Such attempts have generally not been successful, principally because of the incompatibility of silicones with many thermoplastics.